The domain "server1.example.com" (or your replacement for it) has to exist in DNS and must point to your server. So thats not a problem with your server setup, its a problem with the dns records of your domain.

You selected that during ispconfig install. If you would have selected to create no ssl for ispconfig, then the access would be http://

Also how can I change the host name like thisismycompany123.com

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You set the hostname during install of your server. The hostname has to be a subdomain like server1.thisismycompany123.com and not thisismycompany123.com. If you would use thisismycompany123.com, then you wozld not be able to have any email addresses for the domain thisismycompany123.com.